Photovoltaic devices have been receiving attention as a clean energy source. There are two types of representative photovoltaic devices: those using an element made of a crystal silicon semiconductor wafer; and those using a semiconductor layer made of amorphous silicon. The former requires complicated steps to produce a single-crystal ingot and work the single-crystal ingot into semiconductor wafers. Further, its production cost is high, since the utilization rate of expensive silicon raw material is low due to the crystal waste produced in the cutting step or the like. In the latter, when the amorphous structure, in which hydrogen is combined to the dangling bond of silicon, is exposed to light, the hydrogen is released and a structural change tends to occur. Hence, the latter device has a problem in that the photovoltaic efficiency lowers gradually due to exposure to light.
In order to provide a photovoltaic device that is free from such degradation of characteristics, inexpensive, and expected to provide a high output, there have been examined spherical solar cells using a spherical photovoltaic element that is composed of a spherical p-type semiconductor (first semiconductor) and an n-type semiconductor layer (second semiconductor layer) formed on the surface of the spherical p-type semiconductor. One such example is a solar array as proposed in U.S. Pat. No. 4,581,103, in which spherical silicon (Si) elements are embedded in the apertures of a flat aluminum (Al) foil, their n-type semiconductor layers are etched from the backside of the aluminum foil to expose the interior p-type semiconductors, and the exposed p-type semiconductors are connected to another aluminum foil.
This proposal utilizes small elements with a diameter of around 1 mm, thereby decreasing the average thickness of the whole photovoltaic section and reducing the amount of high-purity Si. However, since this spherical solar cell is of the type not utilizing reflected light, its output per element is low. Thus, in order to improve the conversion efficiency per light-receiving surface of the module, it is necessary to arrange a large number of elements densely such that they are close to one another. Hence, the process for connecting the elements to the aluminum foil becomes complicated and, moreover, a large number of elements are necessary. As a result, a large cost reduction is not possible.
Also, in order to bond the Si semiconductors to the aluminum foil conductor layer and obtain a good electrical connection therebetween, the above proposal includes the step of applying a heat treatment at 500 to 577° C. to form an alloy layer of aluminum and Si at the joint therebetween. However, such a heat treatment causes a short-circuit phenomenon since the second semiconductor layer is a thin layer with a thickness of 0.5 μm or less and the conductor layer pierces the thin second semiconductor layer upon the heat treatment. Therefore, there are drawbacks of large deteriorations in open-circuit voltage, fill factor, and the like.
In order to solve these problems, there has been proposed a solar cell that is composed of a support with a large number of recesses and spherical photovoltaic elements with a diameter of around 1 mm disposed in the recesses, wherein the inner faces of the recesses are utilized as reflecting mirrors (e.g., Japanese Laid-Open Patent Publication No. 2002-50780, US 2002/0096206 A1, and US 2004/0016456 A1). Such a solar cell is called a micro concentrator-type or low concentrator-type spherical solar cell. A first advantage of this configuration is a reduction in the amount of element materials, particularly, expensive Si. A second advantage is effective utilization of light; due to the action of the reflecting mirror, light that is 4 to 6 times as much as the light directly incident on the element can be allowed to enter the element.
As a representative conventional method for producing photovoltaic devices of this type, the proposal previously made by the present inventors (US 2004/0016456 A1) is described below. A photovoltaic element comprises a spherical first semiconductor and a second semiconductor layer covering the surface thereof, and a part of the first semiconductor is exposed from the second semiconductor layer. An electrode is previously formed on the exposed part of the first semiconductor and the second semiconductor layer. A support has a plurality of recesses for receiving such elements and comprises a second conductor layer to be electrically connected to the second semiconductor layers and an electrical insulator layer formed on the backside of the second conductor layer. On the backside of the electrical insulator layer is formed a first conductor layer that electrically interconnects the electrodes of the first semiconductors.
According to such a configuration, electrode formation requiring a high-temperature heat treatment is done before the elements are disposed in the support. Hence, there is an advantage in that the electrodes and the conductor layer can be connected at a relatively low temperature after the elements are disposed in the support. However, since the electrode for the second semiconductor layer is formed on a curved surface near the opening of the second semiconductor layer, such electrode formation requires accurate positioning and a technique for forming a fine shape, thus being unsuitable for mass-production.
Also, this support has a two-layer structure consisting of the second conductor layer with the recesses for receiving the elements and the electrical insulator layer. Such a support can be produced, for example, by integrally laminating a second conductor layer, which is obtained by working a metal sheet to form a plurality of recesses, each recess having an aperture in the bottom, and an electrical insulator sheet with through-holes corresponding to the apertures. However, in actuality, in the process of integrating these two sheets by adhesion or thermo-compression bonding, the electrical insulator sheet made of resin may be deformed, thereby causing changes in the pitch, dimensions, and shape of the through-holes and hence displacement. It is therefore difficult to produce such a support with good accuracy. The three-layer support as disclosed, for example, in US 2002/0096206 A1 also has the same problem as the above-mentioned two-layer support.
Further, in spherical solar cells, it is extremely important to mount all of the large number of very small spherical elements in predetermined positions inside the small recesses accurately and promptly. If the positioning of the elements is inaccurate or dislocation occurs during the production process or operation of the spherical solar cell, the second conductor layer comes into contact with the exposed part or electrode of the first semiconductor to cause a short-circuit, or an electrical connection cannot be formed between the semiconductor and the conductor layer. In the event of separation of the elements, the output of the photovoltaic device lowers.
In order to solve these problems, for example, US 2004/0016456 A1 discloses a method of positioning elements, with a conductive paste applied to the electrodes of their second semiconductors, inside the recesses of the support, and then heating the elements to fix them to the support. However, this method has following problems. For example, it is difficult to apply the conductive paste to the minute electrodes at a high speed. Also, in the process of positioning the elements in the support, the conductive paste applied to the elements adheres to the reflecting mirrors of the recesses, thereby lowering the photovoltaic efficiency.